The simultaneous or sequential injection (coinjection) of two or more resins into a mold cavity to develop layered wall structures is well known as evidenced by the disclosures of U.S. Pat. Nos. 4,497,621, 4,518,344, 4,525,134, 4,526,821, 4,609,516 and 4,657,496.
These disclosures, while providing separate flow paths for the respective resins, do not show or suggest a method for maintaining each resin at its optimum processing temperature during transit from an extruder or other source to the mold cavity.
The maintenance of processing temperatures of each individual resin is especially important when the optimum processing temperature of one resin causes degradation of a second resin or vice versa.
For example, it is frequently desirable to mold a layered wall structure for a hollow article or a preform where coinjection materials include ethylene vinyl alcohol copolymer (EVOH) which processes most satisfactorily at temperatures ranging from 400.degree. to 440.degree. F. and polyethylene terephthalate (PET) which processes best at temperatures ranging from 500.degree. to 550.degree. F. As stated previously, failure to maintain these optimum temperatures individually leads to degradation of one resin or the other resulting in a defective product.
In prior art machines and processes numerous procedures have been devised to minimize the deleterious effect arising from resins having wide ranging and different processing temperatures.
Some procedures involve attempting to process the resins very quickly minimizing residence time and thus minimizing degradation.
Other procedures involve the use of resins that process at generally the same temperature range. Obviously, such a procedure limits the choice of resins and precludes their selection based upon desired barrier or other physical properties.
While the above coinjection methods and procedures are operable, it is highly desirable to enhance commercial success to provide coinjection molding machines and processes in which there is complete freedom to choose resin materials on the basis of their barrier characteristics even though the chosen resins process at different temperatures.
A particular complication arises if three or more resins are utilized, each from separate sources, especially where each should be maintained at separate temperatures.
The aforesaid copending U.S. patent application Ser. No. 862,269 contemplates the coinjection of two or more resins; however, it is desirable to provide an improved process and apparatus for the tri-injection of three or more resins accommodating the complexities of such a system where each resin should be maintained at separate temperatures.